1. Field of the Invention
The present invention relates to a PRML regenerating apparatus making use of partial-response signaling with a maximum-likelihood sequence detection.
2. Description of the Related Art
A PRML (Partial-Response Signaling with Maximum-Likelihood Sequence Detection) has been utilized for enhancing a recording density of the magnetic disc and a magneto-optic disc apparatus in recent years. In such a PRML system, a PRML regenerating apparatus for regenerating a read signal is provided.
In a disc storage apparatus making use of the partial-response signaling, the regenerating apparatus is constructed of a waveform equalizing circuit and a maximum-likelihood decoder. A receiving filter of the waveform equalizing circuit of this regenerating circuit equalize-shapes an output signal of a record channel into a partial-response signal. Then, the maximum-likelihood sequence detector (maximum-likelihood decoder) performs, after making a ternary determination of the equalizing signal, the maximum-likelihood detection and thus reconstructs a recorded data string.
This type of PRML regenerating apparatus is disclosed in the specifications of U.S. Pat. No. 5,060,088, U.S. Pat. No. 4,644,564, U.S. Pat. No. 4,707,681, U.S. Pat. No. 4,786,890 and U.S. Pat. No. 4,888,775.
In the conventional PRML regenerating apparatus, parameters of the waveform equalizing circuit and the maximum-likelihood decoder are set to fixed values when delivered from the manufacturer of the apparatus. Therefore, characteristics of the waveform equalizing circuit and the maximum-likelihood decoder are also fixed. For example, in a ternary determination circuit of the maximum-likelihood decoder, a distance between two slice levels for slicing the input signal is fixed.
As a matter of fact, however, a sample signal quality is deteriorated due to a defect on a magnetic medium. Further, there is also the deterioration in terms of the sample signal quality in a case where a signal-to-signal interference described by a polynomial (1-D) can not be quantitatively controlled due to an equalization error. In addition, there exists the deterioration of the signal quantity that is derived from a scatter in terms of characteristics of an MR (magneto-resistive) head. If the equalization error occurs because of the characteristics of the above head, the magnetic medium and the waveform equalizing circuit, there arises such a problem that effective maximum-likelihood decoding can not be executed according to the prior art in which the distance between the above slice levels is fixed.
Besides, if the characteristics of the head and the waveform equalizing circuit are not proper, a problem is caused, wherein the equalization error is often produced, and an optimum regeneration can not be performed.
Further, the conventional PRML regenerating apparatus presents a problem in which a configuration thereof is complicated.